Crude petroleum (also called “crude oil” or “crude”) is composed primarily of hydrocarbons of paraffin, naphthene, and aromatic types: each of these groups contains a broad range of molecules and hence, the composition and properties of the crude can vary significantly. Most jet fuels are made by refining crude petroleum under carefully controlled process conditions.
Processes for the production of jet fuel are known to the person skilled in the art and many technical descriptions are available (see for example, “Aviation Fuels—Technical Review”, FTR-3 (2005), published by Chevron Texaco).
In brief, the refining process can generally be considered to comprise four stages of treatment. First, separation processes (most commonly, distillation), in which the crude is separated into two or more components based on a physical property, such as boiling point. Secondly, upgrading processes (for example, “sweetening”, hydrotreating and clay treating), which improve the quality of the material by using chemical reactions to remove undesirable (trace) compounds. A standard sweetening process used in the production of jet fuels is Merox™ treatment. Physical processes such as filtering/coalescing to remove particulate matter and/or water contamination can be considered to be a third category of treatment. The fourth stage relates to conversion processes (such as catalytic cracking and hydrocracking), in which the molecular structure of the feedstock is changed, for example, by “cracking” (breaking down) large molecules into smaller more desirable molecules. In addition, depending on the intended use of the hydrocarbon product, one or more chemical additives may be added so that the fuel meets any relevant quality specifications.
Fuel additives are now considered almost essential for meeting the strict quality and performance parameters (for example, oxidation stability requirements) set for jet fuels. In fact, fuel additives may be used for a number of important applications, for example: antibacterial agents are useful for preventing the growth of microorganisms that may otherwise break down the fuel and/or block fuel lines; anti-icing chemicals may be used to prevent the freezing of any water contamination when the fuel is exposed to low temperatures (e.g. at high altitudes); antioxidants may be used to prevent dissolved oxygen from setting off chains of oxidation reactions and, thus, to improve storage stability; metal deactivator agents may be used to chelate metals, such as copper and zinc, that may catalyse oxidation reactions; corrosion inhibitors are useful for preventing the corrosion of storage tanks and distribution pipelines by oxygen and water in the fuel; and electrical conductivity agents (e.g. anti-static and/or static-dissipater additives) are often included to maintain a minimum electrical conductivity, which helps prevent static charge build up. Suitable additives for use in jet fuel are known to the skilled person in the art, and are discussed, for example, in Aviation Fuels: Technical Review (FTR 3), 2005, Chapter 4, Edited by Chevron Texaco.
Jet fuel, such as Aviation Turbine Fuel (Jet A1), is subject to particularly strict quality specification parameters and, therefore, jet fuel must be “certified” before it can be accepted for its intended use. The required fuel specifications may vary according to national or institutional requirements and the intended use. Hence, the various parameters defining the quality of jet fuel are published and updated periodically, most notably by The Director, Defence Fuels Group, UK, under the title Defence Standard 91-91 (DEFSTAN 91-91), and these parameters are well known to those who deal with the fuel. Updated information on jet fuel specification parameters in force in different parts of the world may also be periodically published by commercial organisations such as, for example, ExxonMobil Aviation International Ltd.
A refined hydrocarbon liquid composition (such as a jet fuel) is typically tested at the refinery to ensure that it meets the appropriate specification parameters before it is distributed. However, the distribution chain from the refinery to the point at which the product is used may be both convoluted and slow. For example, a jet fuel may be pumped or transported to one or more fuel terminals (e.g. by ocean tanker or pipeline) for intermediate storage, before it is delivered to an airport terminal, where it also may be stored until it is finally delivered into an aircraft. Such distribution chains can mean that an already certified fuel is exposed to various sources of possible contamination (for example, when transported through pipes or in tanks that have been used for different fuels), and exposed to variable temperatures over prolonged periods of time, which may cause fuel degradation.
It is known that fuel properties can deteriorate significantly during the distribution (and storage) chain, and this deterioration can lead to significant operational difficulties downstream of the refinery, even though the fuel may still fall within the specification parameters. For example, surfactants, ionic species (such as metal ions), particulate matter and water can all build up in fuel during its storage and transportation. Surfactants can occur naturally in crude oils (e.g. naphthenic acids), they may be introduced during the refining process, or may be picked up from contaminated pipes and containers. Ionic species, such as metal ions/oxides and particulate matter are typically introduced through the deterioration (e.g. corrosion) of metal fuel transportation pipelines and storage tanks. Particulate matter can originate from the corrosion of pipelines, storage tanks, and through chemical reactions taking place in the fuel, or by the action of microorganisms. Water can be absorbed through contact with air and by the transportation of fuel; for example, containers and pipes may be washed with water between different shipments. All of these contaminants lead to premature “disarming” (i.e. inactivation) of water filters, which are used, for example, at airports to remove water from jet fuel immediately before it is loaded into an aircraft. It is, therefore, very important to minimise the build up of such contaminants, or to remove the contaminants prior to the final water filtration step, to: (i) prevent the transmission of unacceptably high levels of water through the filter; and (ii) extend the service life of the filter.
At least a part of the cause for the degradation of fuel during storage and transportation and the resulting build up of contaminants is recognised to be the presence of sulfur, oxygen and nitrogen based compounds in petroleum fractions. These compounds, in particular sulphur-based compounds are also undesirable from an environmental standpoint. Hence, refineries and associated downstream facilities have developed a number of processes that are intended to remove or minimise detrimental materials, so that the quality of the fuel end-product is maintained within the original specification limits.
For example, U.S. Pat. No. 2,090,007 (August 1937), describes a process for treating cracked gasoline to meet marketing requirements by using chemicals such as an acid or an alkali and then passing the fuel through a column containing Fuller's Earth or charcoal as a medium to remove the impurities and improve the quality of the resulting product. This patent outlines the utilisation and abilities of attapulgus clay to remove unwanted impurities in the gasoline fraction of fossil fuel. Such clay treaters are employed routinely in the initial refining of crude petroleum.
U.S. Pat. No. 3,529,944 (September 1970) relates to a process for refining fuel fractions, such as jet fuels, in particular to improve thermal stability. The process described involves the addition, at the petroleum refinery, of degradation accelerators in order to catalyse the formation of particulate impurities, followed by treatment (for example, at a fuel depot) with a solid, particulate adsorbent media such as natural or synthetic clays, fuller's earth, attapulgite and silica gel to removed the contaminants. Finally, it is necessary to reintroduce stabilising chemicals, such as antioxidants, into the treated fuel to retard any further degradation of the fuel before use.
Another identified cause for the degradation of fuel during storage and transportation is contamination of the refined fuel with catalysts and by-products of the Merox™ process, which is routinely used for demercaptanisation of crude petroleum.
U.S. Pat. No. 6,579,444 (June 2003) relates to a process for removal of sulfur compounds from a hydrocarbon stream. The adsorbent material comprises of cobalt and one or more group VI metals selected from molybdenum and tungsten on an inorganic refractory support, which is chosen from alumina, silica or large pore zeolites. This patent identifies some of the problems associated with Merox™ treating of hydrocarbons, such as sodium and water contaminated product streams; and the non-removal of sulfur compounds such as sulfides and thiophenes, oxygen compounds such as phenols, phenolates and peroxides, and nitrogen compounds such as amines or nitrates.
An alternative to Merox™ treatment for demercaptanisation of petroleum distillates such as gasoline, kerosene and diesel fractions is described in U.S. Pat. No. 6,485,633 (November 2002). The demercaptanisation in this case is achieved by sorption of the mercaptans with activated carbon. This patent identifies the possible contamination of the refined distillate streams with the remnants of the catalysts used in demercaptanisation processes such as the Merox™ processes.
U.S. Pat. No. 6,422,396 identifies the problem of rapid disarming of conventional water separators due to inter alia surfactants in refined hydrocarbon streams. The patent describes a coalescer (water filter) for use in hydrocarbon streams that contain surfactant, and which is intended to separate a discontinuous phase of water from a continuous phase of hydrocarbons, such as jet fuel. In addition, the patent discusses the unsuitability of alternative methods for preventing the disarming of water separators, such as upstream use of a clay treater. Further in this regard, it is stated that the use of a clay treater is inappropriate, because a clay treater also removes desirable surfactants in the fuel.
The Coordinating Research Council (CRC) in its report no. 552 (February 1987) studied the removal of surfactants from jet fuel using activated clay elements. However, the study suggested methods of testing such as ASTM D 3602/ASTM D 3630, which have become obsolete and no longer in use. Furthermore, it is notable that the use of ASTM D 3948 suffers from a lack of reproducibility at WSIM ratings (i.e Water Separometer Index, Modified rating also known as MSEP—Micro Separometer—rating) of around 90, which has a detrimental effect on the precision of these tests. In addition, this study suggests that if the WSIM or MSEP rating drops to about 90 or below then the clay bed starts to deteriorate. Therefore, the method described may not be applicable for jet fuels that may have WSIM ratings as low as 70.
Accordingly, there is a need for a means of treating a hydrocarbon liquid composition to improve fuel quality by eliminating or at least alleviating the problem of contaminant build up during the storage and transportation of the hydrocarbon liquid composition, and which means does not result in undesirable side-effects or by-products. In particular, there is a need for a method or process for treating a hydrocarbon liquid composition in order to extend the service life of water filters, for example, at airports; and to prevent or at least reduce the transmission of unacceptably high levels of water through a water filter.
Another problem associated with the storage and transportation of certified fuels is that the conductivity level of the fuel can drop significantly over time, to below optimal levels. Moreover, the previously discussed problems caused by the reduced efficiency of water removal from fuel can be exacerbated due to this decrease in fuel conductivity during storage and transportation. In this regard, static dissipaters or anti-static additives (for example, ASA-3 and Stadis® 450), which are added to fuel to maintain acceptable conductivity levels, also act as weak surfactants, which can increase the difficulty of removing water from fuel.
This is known to be a particular problem with non-hydrotreated fuels (e.g. Merox™-sweetened fuels). In this case, it has been found that the interaction of the static dissipater compound with the non-hydrotreated fuel can result in a rapid deterioration in the conductivity of the fuel; however, despite this effect the MSEP or WSIM rating surprisingly does not rise proportionally. Hence, any further addition (redoping) of static dissipater to counteract the drop in conductivity causes a further drop in water separability (i.e. a lower MSEP rating), potentially to levels below the acceptable minimum value of 70 when static dissipaters are used. This problem is most significant when Stadis® 450 is used, because it causes a greater proportional reduction in MSEP rating than does ASA-3 (CRC Report No. 601; The Effect of Stadis 450 on MSEP Rating and Coalescence—Technical Basis for Re-doping Turbine Fuels with Stadis 450; July 1996; Coordinating Research Council Inc., Georgia, USA). Although this report suggests that Stadis® 450 may not directly disarm coalescers (despite the measured reduction in MSEP rating), the report highlights concerns over the levels of contamination that can build up during the transportation and storage of fuels.
Hence, it would also be desirable to have a method or process for treating a hydrocarbon liquid composition, such as a jet fuel, which prevents or at least alleviates the problems associated with loss of conductivity during storage and transportation. In this way, any redoping of static dissipater or anti-static compounds that may be required to achieve the desired conductivity level may be eliminated or at least reduced.
It would be further desirable to have a method or process for treating a hydrocarbon liquid composition, such as a jet fuel, which prevents or at least alleviates some of the problems associated with a reduction in water separability (as indicated by a reduced MSEP rating), that can result during storage and transportation, and particularly as a result of the redoping of static dissipater or anti-static compounds to maintain the desired conductivity level.
This invention aims to address at least some of the above problems in the prior art, by providing a process that improves the quality of hydrocarbon liquid compositions such as fractions of petroleum crudes, condensates and compositions including petroleum distillates, naphthas, gasolines, kerosenes, jet fuels, diesel fuels, fuel oils and the like, which can deteriorate from specification parameters during storage and transportation (shipment), and consequently lead to operational difficulties that affect the performance of supply chain management downstream of the refinery.
It is thus an object of this invention to ensure improvement in quality of petroleum products, and particularly Merox™-treated jet fuels. Another object of this invention is the use of clay treaters as novel polar-exchangers to remove from the hydrocarbon liquid compositions, sub-ppm levels of impurities whose presence can cause a deterioration of quality parameters on storage and transportation, but which does not adversely affect the conductivity of the hydrocarbon liquid composition. These and other aspects, objects and the benefits of this invention will become clear and apparent on studying the details of this invention and the appended claims.